A U.S. Army project discovery has upended previous notions about how metals deform and could help guide the creation of stronger, more durable materials for military vehicles.
For nearly 100 years, scientists thought they understood everything there was to know about how metals bend. They were wrong.
Materials science and engineering researchers at the University of Wisconsin-Madison, funded by the Army Research Office, demonstrated that the rules of metal bending aren’t so hard-and-fast after all. The researchers’ new mechanism for bending, published in Nature Communications, might allow engineers to strengthen a material without running the risk of fractures.
The Army Research Office is an element of U.S. Army Combat Capabilities Development Command’s Army Research Laboratory.
“This creates new opportunities for materials design,” said Izabela Szlufarska, a professor of materials science and engineering at UW-Madison. “It adds another parameter we can control to enable strength and ductility.”
“Professor Szlufarska has opened up an entirely new area for exploration for structural materials processing and design,” said Dr. Michael Bakas, synthesis and processing program manager at Army Research Office. “By making such a high impact discovery, Professor Szlufarska has potentially laid the technical foundation for the development of a new generation of advanced structural materials that could eventually be employed in future Army equipment and vehicles.”
Currently, engineers manipulate the strength of a metal through techniques such as cold working or annealing, which exert their effects through small, yet important, structural irregularities called dislocations.
“Everybody in the metals community knows that dislocations are critical,” Szlufarska said.
It’s a truism that’s held since 1934, when three researchers independently realized that dislocation explained an ages-old paradox: Metals are much easier to bend than their molecular structures–which typically take the form of regularly repeating three-dimensional grids–would suggest.
Dislocations are tiny irregularities in the otherwise well-ordered crystal lattice of a metal. They arise from slight mismatches–picture the pages of a book as rows of atoms, and imagine how the neat stack of paper becomes ever-so-slightly distorted at the spot where someone inserts a bookmark.
Normal metals bend because dislocations are able to move, allowing a material to deform without ripping apart every single bond inside its crystal lattice at once.
Strengthening techniques typically restrict the motion of dislocations.
So, it was quite a shock when Szlufarska and colleagues discovered that the material samarium cobalt, known as an intermetallic, bent easily, even though its dislocations were locked in place.
“It was believed that metallic materials would be intrinsically brittle if dislocation slip is rare,” said Hubin Luo, University of Wisconsin-Madison staff scientist. “However, our recent study shows that an intermetallic can be deformed plastically by a significant amount even when the dislocation slip is absent.”
Instead, bending samarium cobalt caused narrow bands to form inside the crystal lattice, where molecules assumed a freeform “amorphous” configuration instead of the regular, grid-like structure in the rest of the metal.
Those amorphous bands allowed the metal to bend.
“It’s almost like lubrication,” Szlufarska said. “We predicted this in simulations, and we also saw the amorphous shear bands in our deformation studies and transmission electron microscopy experiments.”
A combination of computational simulations and experimental studies was critical to explaining the result.
Next, the researchers plan to search for other materials that might also bend in this peculiar manner. Eventually, they hope to use the phenomenon to tune a material’s properties for strength and flexibility.
“This might change the way you look for optimization of material properties,” Szlufarska said. “We know it’s different, we know it’s new, and we think we can use it.”
“While there is a long road of further research ahead, Professor Szlufarska’s discovery certainly opens up new avenues to explore for extramural and Army researchers working in structural materials,” Bakas said. “It is this type of fundamental research discovery that eventually results in the advanced materials that will protect soldiers 25 to 30 years from now.”